The present invention lies in the art of refining platinum group metals (PGM's) through solvent extraction. More specifically, the invention relates to a process for purifying alkyl sulfide extractant for use in extracting palladium from a mixture of palladium and other PGM's. In particular, the process is directed to the removal of organosulfur compounds from alkyl sulfide extractant, as they interfere with the recovery of palladium.
Solvent extraction of aqueous solutions containing precious metals derived from ores and their subsequent products is gaining in use due to its advantages over the classical precipitation and redissolution methods. Advantages of solvent extraction include a reduced cycle time, higher purity, and often higher yields. With proper selection of solvents and process conditions, extraction can be highly specific for a particular metal species, resulting in recovery of a high purity product. Typically, the required purity of PGM's is up to about 99.995%. To achieve this level of purification using precipitation and redissolution procedures requires multiple repeat processing and thus considerable time. Solvent extraction shortens the time for purification many fold, while yielding a higher purity product.
Solvent extraction is carried out by contacting an aqueous phase solution of one or more extractable metals with an organic phase containing the extractant. The extractable metal or metals form complex compounds, with the extractant acting as the complexing agent or ligand. Each of the extractable metals can form one or more complexes with different atomic or molecular entities occupying ligand positions in the coordination spheres of the complexes. These ligands can be anions (chloride and other halides, sulfate, nitrite), cations (nitrosyl, NO.sup.+), neutral species (H.sub.2 O, NH.sub.3) or organic compounds (ethers and other oxygenated compounds, amines, sulfides). For those metals exhibiting more than one valence state, some of which can be easily reduced (e.g. Au(III), Pd(IV) and Ir(IV)), care must be taken in achieving the desired chemistry for extraction and stripping. Large differences in rates of reduction and ligand substitution reactions result in separation between metal species. Sequential extractions using different extractant enables separation of a plurality of metal species from a single solution.
For recovery of palladium (Pd) from ores containing Pd and other PGM's, various alkyl sulfides have been used as extractant. For such an example, see U.S. Pat. No. 3,985,552 issued Oct. 12, 1972 to Edwards. In a typical extraction, a diluent such as a paraffinic or aromatic liquid is combined With the alkyl sulfide to form an organic phase. An aqueous phase, usually an aqueous acidic chloride solution containing Pd and other metals extracted from the ore, is contacted with the organic phase. The Pd in the aqueous phase forms a complex with the extractant and, due to its high solubility in the organic phase, the Pd complex is almost completely partitioned into the organic phase. Using n-octylsulfide (NOS) as the extractant, for example, the equation for extraction is: EQU PdC;.sub.4.sup.2 (aq.)+2NOS(org.)---CL.sub.2 Pd(NOS).sub.2 (org.)+2Cl(aq.)
In typical commercial practice, the organic phase is first washed with a dilute HCL solution to remove entrained raffinate comprising other PGM's and small amounts of other metals, such as iron and copper, which are extracted to a low level. The washed, loaded organic phase, containing less contaminant metals, is then stripped using aqueous ammonia The Pd complexes with the ammonia to form Pd(NH.sub.3).sub.4.sup.2+ ion in aqueous solution. The extractant is then recycled for reuse. The aqueous Pd solution, which also contains excess NH.sub.3 and Cl, is then carefully acidified to precipitate the sparingly soluble salt Cl.sub.2 Pd(NH.sub.3).sub.2. Conversion to the metal results by its ignition at high temperature (e.q., about 900.degree. C.) to form "Pd sponge."
The extraction and stripping of PGM's using alkyl sulfides is affected by organosulfur impurities such as mercaptans and disulfides present in the alkyl sulfides. During stripping the organosulfur impurities readily react with palladium and other PGM's to form a solid emulsion-like "crud" phase which disrupts the complete recovery of Pd. For example, sulfur compounds, such as mercaptans, are undesirable in the alkyl sulfide as they can tie up metals as the mercaptides and can subsequently cause problems in the stripping step. These problems can be overcome by using highly purified alkyl sulfides, but at considerable added materials cost.
There is a need for a process for conveniently and efficiently purifying the alkyl sulfides which are used for purifying palladium and other PGM's, of the unwanted impurities. This need is met by the present invention.